gnu: gtkmm: Update to 3.16.0.

* gnu/packages/gtk.scm (gtkmm): Update to 3.16.0.

gnu/packages/gtk.scm

@@ -673,7 +673,7 @@ toolkit.")
 (define-public gtkmm
   (package
     (name "gtkmm")
-    (version "3.14.0")
+    (version "3.16.0")
     (source (origin
              (method url-fetch)
              (uri (string-append "mirror://gnome/sources/" name "/"
@@ -681,7 +681,7 @@ toolkit.")
                                  name "-" version ".tar.xz"))
              (sha256
               (base32
-               "12z4g2in82nk92nfjs2hmrdcwbav8v3laz1813x2dhkf5jk2ixfr"))))
+               "036xn22jkaf3akpid7w23b8vkqa3xxqz93mwacmyar5vw7slm3cv"))))
     (build-system gnu-build-system)
     (native-inputs `(("pkg-config" ,pkg-config)))
     (propagated-inputs